Switches are needed in optical communication networks, for, e.g., routing signals and adding and substituting signal-carrying lines. Integrated optical switches are advantageous for these purposes because they can be made more compact than mechanical switches, and can be embedded in integrated networks of optical interconnections.
One integrated optical switch of current interest is based on a Mach-Zehnder interferometer (MZI) that includes a thermo-optic phase shifter in one arm. Light enters the MZI through one branch of an input directional coupler, and exits the MZI through one of alternate branches of an output directional coupler. When a heating current is applied to the phase shifter, the output signal shifts from one to the other of the alternate branches. This switch has excellent transmission characteristics only when the power coupling ratios of the directional couplers are close to 3 dB. When these ratios deviate substantially from 3 dB, the output channels suffer crosstalk and excess loss. The power coupling ratio is close to 3 dB only within a relatively narrow band of wavelengths. As a consequence, effective operation of the switch is limited to a relatively narrow spectral range. For example, a typical switch designed to operate at 1.4 .mu.m will have cross talk, in its powered state, of less than -15 dB over a range of about 1.36 .mu.m-1.45 .mu.m. Significantly, such a range is too small to include both the 1.3-.mu.m and 1.55-.mu.m communication channels.
Some practitioners have modified the MZI switch to extend its useful spectral range. For example, T. Kitoh et al., "Novel Broad-Band Optical Switch Using Silica-Based Planar Lightwave Circuit," IEEE Photon. Technol. Lett. 4 (1992) 735-737, reports a switch in which the directional couplers are replaced by 3-dB wavelength-insensitive couplers (WINCs). Each WINC includes an MZI having a subsidiary thermo-optic phase shifter in one arm. This switch has a predicted crosstalk of less than -20 dB over a spectral range of 1.27-1.62 .mu.m. However, this switch requires relatively high electric power to operate the three thermo-optic phase shifters.
Thus, practitioners in the field have, until now, failed to provide an optical switch that can be operated at relatively low power over a relatively broad spectral band.